In the prior art, there are two methods for fixing a casing to the frame body. In the first design, the casing is fixed to the frame body, and the casing has a door. The user can open the door for placing objects into the casing or taking objects from the casing. In another design, the casing is made as a drawer which can be pulled out from a casing positioned in the frame body for fixing the drawer or pushing into the casing. However, this design still have a complicated structure and thus it is expensive. Moreover, the operation is inconvenient.